1. Field of the Invention
This invention relates generally to formation fluid testing and collection apparatus and more particularly to a modular formation tester having a formation fluid collection system that collects formation fluids at a predetermined pressure and maintains the collected fluid pressure at such pressure throughout the logging operations.
2. Description of the Related Art
In the oil and gas industry, wireline formation testing tools have been used for monitoring formation pressures along a wellbore, obtaining formation fluid samples from the wellbore and for predicting performance of reservoirs around the wellbore. Such formation testing tools typically contain an elongated body having an elastomeric packer that is sealingly urged against the zone of interest in the wellbore to collect formation fluid samples in storage chambers placed in the tool.
During drilling of a wellbore, a drilling fluid ("mud") is used to facilitate the drilling process and to maintain a hydrostatic pressure in the wellbore greater than the pressure in the formations surrounding the wellbore. The drilling fluid penetrates into or invade the formations for varying radial depths (referred to generally as the invaded zones) depending upon the types of the formation and drilling fluid used. The formation testing tools retrieve formation fluids from the desired formations or zones of interest, test the retrieved fluids to ensure that the retrieved fluid is substantially free of mud filtrates and collect such fluids in one or more chambers associated with the tool. The collected fluids are brought to the surface and analyzed to determine properties of such fluids and to determine the conditions of the zones or formations from where such fluids have been collected. It is, therefore, critical that only uncontaminated fluids are collected in the same condition in which they exist in the formation.
However, the prior art formation tester tools typically collect the retrieved formation by transferring such fluids from a probe into one or more chambers with restrictions to slow down the fluid flow rate into such chambers at nearly atmospheric pressure. Frequently water cushions are utilized to fill the chambers more uniformly. In order to not allow the fluid entering into the chamber to flash or to reduce the chances of vaporizing any liquid gas, the formation fluid is pumped into the chamber at a relatively slow rate. Still, it is common for the collected fluid to contain vaporized gas. Additionally, asphaltenes are commonly present in the hydrocarbons and if the pressure in the chamber remains at a relatively low pressure, such asphaltenes tend to flocculate to form gel-type masses in the fluid. The flocculation process is substantially irreversible. Thus, it is desirable not only to withdraw the formation fluids above the bubble point but also to collect the fluids at a pressure in the chambers that is above the bubble point and above the asphaltene flocculation pressure.
Additionally, the temperature difference between the surface temperature and the formation can exceed several tens of degrees fahrenheit. As the tool is retrieved, the chamber temperature drops, causing the pressure in the chamber to drop accordingly. A substantial pressure drop in the chamber can significantly change the condition of the collected fluid. The ideal condition is that in which the fluid is collected and maintained at a pressure that is above the bubble point pressure and the asphaltene flocculation pressure throughout logging operations.
The present invention addresses the above-noted problems and provides a formation tester in which the uncontaminated formation fluid is collected in a chamber maintained at a predetermined pressure above the bubble point. No water cushions are required to uniformly fill the chambers. The tool also automatically maintains the chamber pressure at the predetermined pressure during the entire logging operation regardless of the change in the temperature surrounding the chamber.